So after trying various configurations, it seems to me that one has two options:
1) The current default for new users:
- half-broken IPv4 due to it going through Carrier-Grade NAT; no public IPv4 address, no port-forwarding, unreliable UDP and so on.
- half-working IPv6, due to the modem not supporting prefix delegation and only giving out a /64 prefix which cannot be subnetted without some painful gymnastics.
- a cable modem with minimal/non-existent support for customization, such as its insistence on having the 192.168.x.1 IP address.
2) The setup that one gets if they ask for "Bridge mode" from UPC support:
- Fully functional dynamic IPv4; which is good.
- No IPv6 at all. None. It's just not there, period; I assume that IPv6 in the default "IPv6 router mode" is implemented using some sort of tunelling, instead of being native IPv6.
- If someone wants fully functional IPv6 (i.e. proper prefix delegation), UPC is not for them.
- If someone wants port-forwarding in IPv4 and at least _some_ IPv6 support, UPC is not for them.
Am I missing something? Are there any plans to improve the IPv6 situation in the near future?